DESCRIPTION: (Applicant's abstract) The focus of this study is on the mechanisms that regulate Purkinje cell number, both during normal development and in pathological conditions. Purkinje cells play a critical role in the regulation of pattern formation during cerebellar development. However, little is known about how Purkinje cell number is regulated. The consequences of Purkinje cell loss can also have profound effects on cerebellar development as in ataxia telangeictasia, a crippling inherited disorder of the immune and nervous systems. The experimental design of this study is to investigate normal and pathological programmed cell death in Purkinje cells using transgenic mice that overexpress a human bcl-2 gene (Hu-bcl-2) as experimental probes. Bcl-2 is a protooncogene that has been shown to protect neuronal and non-neuronal cells from programmed cell death. The initial studies of Hu-bcl-2 transgenic mice have shown that the number of Purkinje cells is increased by as much as 40%. This result suggests that overexpression of bcl-2 is rescuing Purkinje cells from naturally occurring cell death. The two specific aims of this grant proposal are 1) to investigate naturally occurring Purkinje cell death as a mechanism for regulating Purkinje cell number, and 2) to investigate the mechanisms of Purkinje cell loss in staggerer mutants. Naturally occurring Purkinje cell death will be investigated in three experimental protocols. First, the number of Purkinje cells in Hu-bcl-2 transgenic mice will be compared to controls during development to establish when Hu-bcl-2 expression begins to increase Purkinje cell number. Second, the expression of mouse and human bcl-2 will be examined in Hu-bcl-2 transgenics to determine if the transgene is expressed when Purkinje cell number is increased. Third, the cerebella of embryos and neonatal wild type mice will be examined for evidence of Purkinje cell degeneration using molecular probes for degenerating cells. The number of pyknotic Purkinje cells in wild type mice will be compared to Hu-bcl-2 transgenics. The mechanisms of Purkinje cell loss in staggerer mutants will be investigated by searching for evidence of Purkinje cell degeneration.